1. Field of the Invention
This invention relates to the use of solder preforms for attaching leaded components to plated through holes in a printed circuit board and more specifically relates to use of solder preforms which can be used to attach leaded components into through holes in a printed circuit board in a manner which is compatible with conventional surface mount soldering processes.
2. Description of the Prior Art
Conventional techniques for applying leaded components to a printed circuit board include the use of plated through holes and the use of surface mount components. Surface mount components can be soldered to a printed circuit board by conventional reflow processes in which solder or a solder loaded flux is deposited onto a printed circuit board component site. Surface mount components can then be placed into a wet solder paste or an adhesive can be used to hold the component in place. The solder paste is then reflowed using vapor phase, infrared radiation, or convective heating.
Through hole leaded components are typically soldered to a printed circuit board by using a wave soldering process. Conventional wave soldering processes and reflow soldering processes are not compatible, at least in the sense that processes can be simply combined or carried out simultaneously. Various approaches have been suggested in which both leaded and surface mount components can be applied to the same printed circuit board using a reflow process. For example, as suggested in U.S. Pat. No. 4,761,881, a solder paste can be predeposited not only on surface mount pads but also within plated through holes in a printed circuit board. Leads on components can then be inserted into the plated through holes and through the solder paste. This process, however, requires a cure step in which the solder paste on the assembled printed circuit board is baked to remove volatiles from the solder paste prior to the reflow process. Such curing steps can take approximately 45 minutes.
It has also been suggested that a solder paste can be placed within reservoirs formed within the body of a leaded electrical device such as an electrical connector. U.S. Pat. No. 4,641,426 discloses a connector having recesses surrounding leaded pins on the lower surface of a printed circuit board into which a solder paste can be deposited and then cured prior to shipping. These recesses would be defined between stand-offs which comprise integral portions of the electrical connector body.
U.S. Pat. No. 4,767,344 discloses a process in which a solder paste is molded around terminal pins to form premolded beads. The individual solder paste elements are heated within the mold to remove carrier materials with initially disc shaped solder beads ultimately forming teardrop shaped beads.
Each of these processes requires a relatively lengthy curing process. An alternative approach for depositing solder in position to permit leaded components to be soldered to a printed circuit board in a through-hole mounted process is to employ discreet solder preforms, typically in the form of rings or doughnuts. These individual preforms can be placed in surrounding relationship to individual leads between the component and holes in a printed circuit board. For instance, U.S. Pat. No. 4,216,350 discloses a process in which individual solder rings are pressed into apertures in a non-fusible web, either manually or by a automated process prior to positioning the web containing the discreet solder ring in surrounding relationship to pins on a leaded component. This process and other processes which employ discrete solder rings either requires a laborious hand assembly operation in which the rings are individually handled, or an expensive automated process for loading the solder rings or preforms in place. On such apparatus for placing apertured soldered preforms onto pins of an electrical connector is disclosed in U.K. Patent 2,080,262.
Although each of these techniques has certain advantages, each involves a step which is expensive in terms of the time required or in terms of the requirement that expensive automated equipment is necessary. The instant invention in which solder preforms, rings or doughnuts are cast, from pure tin lead solder into an ordered array, eliminates the necessity for lengthy curing of solder paste and also permits a plurality of solder preforms to be handled as a single unit in a single array.